Optical single-bit recording and fluorescent readout utilizing aluminum oxide single crystals

ABSTRACT

The present invention provides methods and apparatuses for writing information to, reading information from, and erasing information on a luminescent data storage medium comprising Al 2 O 3 . The method includes writing and erasing of the information using photoionization via sequential two-photon absorption and non-destructive reading the information using fluorescent detection. The apparatuses for writing and reading the information incorporate confocal detection and spherical aberration correction for multilayer volumetric fluorescent data storage. The methods also allow multilevel recording and readout of information for increased storage capacity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.10/309,021, filed Dec. 4, 2002 now U.S. Pat. No. 6,846,434, entitled,“Aluminum Oxide Material for Optical Data Storage,” which claims thepriority of U.S. Provisional App. No. 60/336,749, filed Dec. 4, 2001,now abandoned, and U.S. application Ser. No. 10/309,179, filed Dec. 4,2002 now U.S. Pat. No. 6,811,607, entitled, “Method for Forming AluminumOxide Material Used in Optical Data Storage,” which claims the priorityof U.S. Provisional App. No. 60/417,153, filed Oct. 10, 2002. The entiredisclosures and contents of the above applications are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods of writing to, reading from anderasing information on a data storage medium.

2. Description of the Prior Art

Various attempts have been made to design higher density data storagemedia for computer devices to replace conventional storage media such asmagnetic disks, CD-ROMs, DVDs, etc. Many of the obstacles faced withrespect to developing improved data storage methods have been associatedwith inadequate storage material properties. For example, photopolymershave been investigated for use in one-bit or holographic data storage.However, photopolymers exhibit strong dimensional shrinkage. Also, mostphoto-sensitive polymers may be used only as WORM media (write once,read many times) and the rewritable photopolymers are still unstable andshow significant fatigue when write-read cycles are repeated many times.Even write-once fluorescent photopolymers show strong reduction offluorescent output signals when read repeatedly. An additional problemwith most photopolymers, as well as for photorefractive crystals,another potential material for volumetric one-bit recording, is thenecessity of using a femto-second high peak power Ti-sapphire laser toachieve efficient two-photon absorption. This type of laser is big,expensive and suitable only for laboratory demonstration.

Therefore, there exists a need for better materials for making highdensity data storage devices.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a datastorage method utilizing an aluminum oxide material that allows for fastelectronic processing in comparison with phase change transition orphoto-induced polymerization techniques for data storage.

It is a further object of the present invention to provide a datastorage method utilizing an aluminum oxide material that is capable ofachieving a write/read rate up to 100 Mbit per second.

It is yet another object of the present invention to provide a datastorage method utilizing an aluminum oxide material that provides theability to perform parallel processing of multiple marks on a datastorage medium for an increase of write/read rate.

It is yet another object of the present invention to provide a datastorage method utilizing an aluminum oxide material that provides highdata storage density restricted only by diffraction limit and NA of theoptical components.

It is yet another object of the present invention to provide a datastorage method utilizing aluminum oxide material that provides thepossibility of multilevel data storage due to dependency of fluorescenceresponse from writing laser energy.

It is yet another object of the present invention to provide a datastorage method utilizing aluminum oxide material that only requires lowlaser light energies for writing and reading of information (pJ and nJrange).

It is yet another object of the present invention to provide a datastorage method utilizing aluminum oxide material that provides extremelyhigh temperature and time stability of stored data.

It is yet another object of the present invention to provide a datastorage method utilizing aluminum oxide material that provides nodegradation of material performance after millions of write/read cycles.

According to a first broad aspect of the present invention, there isprovided a method of writing information to a data storage mediumcomprising the steps of: providing a luminescent data storage mediumcomprising Al₂O₃; and writing the information to the luminescent datastorage medium with an optical source.

According to a second broad aspect of the present invention, there isprovided a method of reading information stored on a data storage mediumcomprising the steps of: exciting a luminescent data storage medium withan optical source to thereby cause the luminescent data storage mediumto emit a fluorescent light signal, wherein the luminescent data storagemedium comprises Al₂O₃ and wherein the optical source emits a read laserbeam having a wavelength in the range of an absorption band of theluminescent data storage medium; and measuring the laser inducedfluorescence light signal from the luminescent data storage medium, tothereby read the information stored on the luminescent data storagemedium.

According to a third broad aspect of the present invention, there isprovided a method of erasing information stored on a data storage mediumcomprising the steps of: providing a luminescent data storage mediumcomprising Al₂O₃, the luminescent data storage medium having theinformation stored thereon; and illuminating the luminescent datastorage medium with an optical source to thereby erase the information.

According to a fourth broad aspect of the present invention, there isprovided a method of writing information to a data storage mediumcomprising the steps of: providing a luminescent data storage mediumcomprising Al₂O₃; and writing the information to the luminescent datastorage medium by using a two-photon absorption technique and aphoto-ionization technique resulting in removal of an electron from acolor center in the luminescent data storage medium and moving theelectron to a thermally stable trap in the luminescent data storagemedium.

According to a fifth broad aspect of the present invention, there isprovided a method of reading information from a data storage mediumcomprising the steps of: exciting a luminescent data storage medium withan optical source having a wavelength in the range of an absorption bandof the luminescent data storage medium to thereby cause the luminescentdata storage medium to emit a fluorescent light signal, wherein theluminescent data storage comprises Al₂O₃ and color centers; andmeasuring the induced fluorescence signal from the luminescent datastorage medium, to thereby read the information stored on theluminescent data storage medium, wherein the method is performed in acondition of one-photon absorption without photo-ionization of the colorcenters and results in excitation of the color centers by the opticalsource to thereby cause the color centers to emit the fluorescencesignal.

According to a sixth broad aspect of the present invention, there isprovided a method of erasing information stored in the data storagemedium comprising steps of: providing a luminescent data storage mediumcomprising Al₂O₃, the luminescent data storage medium having theinformation stored thereon; and illuminating the luminescent datastorage medium with an optical source in conditions of two-photonabsorption to thereby erase the information.

According to a seventh broad aspect of the present invention, there isprovided a apparatus comprising: a luminescent data storage mediumcomprising Al₂O₃; and an optical source for writing information to theluminescent data storage medium.

According to an eighth broad aspect of the present invention, there isprovided a apparatus comprising: a luminescent data storage mediumcomprising Al₂O₃; a first optical source for exciting the luminescentdata storage medium to thereby cause the luminescent data storage mediumto emit a fluorescent light signal when information is stored on theluminescent data storage medium; and means for measuring the emittedfluorescent light signal.

According to a ninth broad aspect of the present invention, there isprovided a apparatus comprising: a luminescent data storage mediumcomprising Al₂O₃; and writing means for writing information to theluminescent data storage medium by using a two-photon absorptiontechnique and a photo-ionization technique resulting in removal of anelectron from a color center in the luminescent data storage medium andmoving the electron to a thermally stable trap in the luminescent datastorage medium, the writing means comprising a first optical source.

Other objects and features of the present invention will be apparentfrom the following detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic diagram illustrating writing information to a datastorage medium of the present invention using a two-photon absorptionand reading information from a data storage medium of the presentinvention using laser-induced fluorescence with a confocal detectionscheme;

FIG. 2 shows optical absorption spectra of two crystals: a known Al₂O₃:Ccrystal used in radiation dosimetry and an Al₂O₃:C,Mg single crystalaccording to a preferred embodiment of the present invention with ahigher concentration of F⁺-centers (absorption at 255 nm) and newabsorption bands corresponding to F₂ ⁺(2 Mg)— and F₂ ²⁺(2Mg)-centersclearly distinguishing a new material;

FIG. 3 is a graph showing the absorption, excitation and emissionspectra of F₂ ²⁺(2Mg)-centers created in Al₂O₃:C,Mg in an as-received orerased state, with an excitation spectrum of 520 nm emission bandcoinciding well with the absorption band at 435 nm assigned to the samedefect, and showing the wavelength dependence of a two-photonphoto-ionization cross-section for F₂ ²⁺(2Mg)-centers;

FIG. 4 is a graph showing a continuous readout of fluorescent signal ata 20 MHz repetition rate with the lifetime of F₂ ²⁺(2Mg)-center emissionat 520 nm equal to 9±3 ns for an Al₂O₃:C,Mg luminescent materialaccording to a preferred embodiment of the present invention;

FIG. 5 is a graph illustrating the multilevel data storage principlebased on the proportionality between 435 and 335 nm absorption bandintensity and the “write” time for a Al₂O₃:C,Mg luminescent material ofa preferred embodiment of the present invention;

FIG. 6 is a graph showing the excitation and emission spectra of F₂⁺(2Mg)-centers obtained as a result of photo-conversion of an F₂²⁺(2Mg)-center (“write” operation) in Al₂O₃:C,Mg crystals using pulsedblue laser light with a wavelength of 430 nm;

FIG. 7 is a graph showing the lifetime measurement of F₂ ⁺(2Mg)-centeremission at 750 nm equal to 80±5 ns for an Al₂O₃:C,Mg luminescentmaterial according to a preferred embodiment of the present invention;

FIG. 8 is a graph showing quadratic dependence of a photo-ionizationcross-section of F₂ ²⁺(2Mg)-centers on peak laser light intensityillustrating high probability of a two-photon absorption process inAl₂O₃:C,Mg crystals;

FIG. 9 is a wavelength dependence of photo-ionization cross-sectionshowing an optimum wavelength at about 390 nm to perform “write”operation in Al₂O₃:C,Mg;

FIG. 10A is a graph showing temperature dependence of an opticalabsorption band at 255 nm (F⁺-centers) and illustrates high thermalstability of trapped charge up to 650° C.;

FIG. 10B is a graph showing temperature dependence of an opticalabsorption band at 335 nm (F₂ ⁺(2Mg)-centers) and illustrates highthermal stability of trapped charge up to 650° C.;

FIG. 10C is a graph showing temperature dependence of an opticalabsorption band at 435 nm (F₂ ²⁺(2Mg)-centers) and illustrating highthermal stability of trapped charge up to 650° C.;

FIG. 10D is a graph showing temperature dependence of an opticalabsorption band at 630 nm (F₂ ³⁺(2Mg)-centers) and illustrating highthermal stability of trapped charge up to 650° C.;

FIG. 11 is a band diagram illustrating electronic processes in an Al₂O₃crystal doped with Mg impurity during “write” and “read” operations;

FIG. 12 is a fluorescent image of a matrix of 3×3 bits written in the XYplane of the crystal medium with 5 μm increments using CW 405 nm laserdiode and read using a confocal detection scheme and 440 nm CW laserdiode;

FIG. 13 is a 500×500 pixel image obtained in fluorescent contrast afterwriting 100×100 bits in the XY plane of the crystal with 1 μm incrementsin which bits were written with pulses of blue 411 nm laser diode andread with 440 nm laser diode;

FIG. 14 is a bit profile obtained in fluorescent contrast during X-scanof a crystal showing spatial resolution of the bits written with 15 nJper bit and 1 μm incremental steps

FIG. 15 is a 400×400 pixel image obtained in fluorescent contrast in theXZ plane of the crystal after writing 3 bits in the X direction with 5μm increments. Bits were written with pulses of blue 411 nm laser diodeand read with 440 nm CW laser diode;

FIG. 16 is a 500×500 pixel image of 3 layers of bits obtained influorescent contrast in the XZ plane of the crystal after writing matrixof 7×3 bits with 5 μm increments in the X direction and 10 μmincremental motion in the Z direction in which bits were written withpulses of blue 411 nm laser diode and read with a 440 nm CW laser diode;

FIG. 17 is a graph demonstrating multilevel one-bit recording bymeasuring a fluorescent image during XY-scan of Al₂O₃ crystal in which10 bits were written in the volume of the crystal with different numbersof 60 ps “write” laser pulses; and

FIG. 18 is a graph illustrating dependence of the modulation depth ofbits as a function of number of “writing” laser pulses and demonstratingmultilevel one-bit recording.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

It is advantageous to define several terms before describing theinvention. It should be appreciated that the following definitions areused throughout this application.

Definitions

Where the definition of terms departs from the commonly used meaning ofthe term, applicant intends to utilize the definitions provided below,unless specifically indicated.

For the purposes of the present invention, the term “writing” refers tothe conventional meaning of the term “writing” with respect to storinginformation on a data storage medium. In a preferred embodiment of thepresent invention, information is written to a data storage medium usinga laser beam of one or more frequencies.

For the purposes of the present invention, the term “write position”refers to positioning a data storage medium to a position at which thedata storage medium may be written to by a laser beam.

For the purposes of the present invention, the term “modulation depth ofa fluorescent signal” refers to the parameter of the optical datastorage system determined as the ratio of two fluorescent signalsobtained from the same media spot before and after recording/writing.

For the purposes of the present invention the term “multilevelrecording” refers to a method of writing information in a storage mediumin which upon readout with a reading beam produces a readout value thatmay be digitized onto several discrete value levels representing morethan one bit of digital information.

For the purpose of the present invention, the term “write time” refersto the time during which the writing beam is illuminating the spot onthe medium to achieve desired change in the fluorescence signalamplitude (modulation depth of fluorescent signal). Such change in thefluorescence signal amplitude may be as low as 1% or as high as morethan 90% depending on the desired modulation depth.

For the purposes of the present invention, the term “reading” refers tothe conventional meaning of the term “reading” with respect toretrieving information stored on a data storage medium. In a preferredembodiment of the present invention, information is read from a datastorage medium using a laser beam of one or more frequencies.

For the purposes of the present invention, the term “read time” refersto the time a specific storage location is illuminated by the readlaser. The read time is equal to the read laser pulse length forstationary media and as a ratio of the reading spot size to the mediumvelocity for moving the medium.

For the purposes of the present invention, the term “erasing” refers toany of the conventional meanings of the term “erasing” with respect todigital data storage media. In general, erasing refers to restoring oneor more sections of a data storage medium containing stored informationto a state those sections had before having information stored in thosesections.

For the purposes of the present invention, the term “physically erasing”refers to removing or destroying previously stored information on a datastorage medium.

For the purposes of the present invention, the term “absorption band inthe region of” or “emission band in the region of” refers to anabsorption or emission band having a peak in the appropriate region.Sometimes the region may be a particular wavelength and sometimes theregion may include a range of wavelengths indicating a possible shift ina band peak position.

For the purposes of the present invention, the term “crystallinematerial” refers to the conventional meaning of the term “crystallinematerial”, i.e. any material that has orderly or periodic arrangement ofatoms in its structure.

For the purposes of the present invention, the term “luminescentmaterial” refers to any of the conventional meanings of the term“luminescent material”.

For the purposes of the present invention, the term “data storagemedium” refers to a medium upon which data may be stored, generally indigital form.

For the purposes of the present invention, the term “luminescent datastorage medium” refers to a data storage medium that is comprised inpart or in its entirety of a luminescent material.

For the purposes of the present invention, the term “defect” refers tothe conventional meaning of the term “defect” with respect to thelattice of a crystal, i.e. a vacancy, interstitial, impurity atom or anyother imperfection in a lattice of a crystal.

For the purposes of the present invention, the term “oxygen vacancydefect” refers to a defect caused by an oxygen vacancy in a lattice of acrystalline material. An oxygen vacancy defect may be a single oxygenvacancy defect, a double oxygen defect, a triple oxygen vacancy defect,or a more than triple oxygen vacancy defect. An oxygen vacancy defectmay be associated with one or more impurity atoms or may be associatedwith an interstitial intrinsic defect such as misplaced interstitialoxygen atoms. Occupancy of an oxygen vacancy by two electrons gives riseto a neutral F-center, whereas occupancy of any oxygen vacancy by oneelectron forms an F⁺-center. An F⁺-center has a positive charge, withrespect to the lattice. A cluster of oxygen vacancy defects formed bydouble oxygen vacancies is referred to as an F₂-type center. A clusterof oxygen vacancy defects formed by two F⁺-centers andcharge-compensated by two Mg-impurity atoms is referred to as a F₂²⁺(2Mg)-center.

For the purposes of the present invention, the term “F-type center”refers to any one of the following centers: F-center, F⁺-center, F₂⁺-center, F₂ ⁺⁺-center, F₂ ⁺(2 Mg)-center, F₂ ⁺⁺(2Mg)-center, etc.

For the purposes of the present invention, the term “color center”refers to the conventional meaning of the term “color center”, i.e. apoint defect in a crystal lattice that gives rise to an opticalabsorption of a crystal and upon light excitation produces a photon ofluminescence. A color center, an impurity or an intrinsic defect in acrystalline material creates an unstable species. An electron localizedon this unstable species or defect performs quantum transition to anexcited state by absorbing a photon of light and performs quantumtransition back to a ground state by emitting a photon of luminescence.In a preferred embodiment of the present invention, color centers arepresent in a concentration of about 10¹³ cm⁻³ to 10¹⁹ cm⁻³.

For the purposes of the present invention, the term “luminescencelifetime” or “fluorescence lifetime” refers to a time constant of anexponential decay of luminescence or fluorescence.

For the purposes of the present invention, the term “wide emission band”refers to an emission band that has full width at half maximum biggerthan 0.1 eV and is a result of strong electron-phonon interaction. Oneexample of a wide emission band is the wide emission band around 520 nm.

For the purposes of the present invention, the term “charge-compensated”refers to a defect in a crystal lattice that electro-staticallycompensates the electrical charge of another defect. For example, in apreferred embodiment of the present invention, Mg and C impurities maybe used to charge-compensate one oxygen vacancy defect, two oxygenvacancy defects, a cluster of these defects, etc. comprising F₂²⁺(2Mg)-centers.

For the purposes of the present invention, the term “two-photonabsorption or 2PA” refers to a quantum mechanical process of lightabsorption by a color center when two photons have been absorbedsimultaneously or sequentially by the localized electron of the colorcenter and the electron makes a quantum transition into an excited stateor a conduction band of the crystal.

For the purposes of the present invention, the term “one-photonabsorption or 1PA” refers to a quantum mechanical process of lightabsorption by a color center when only one photon has been absorbed bythe localized electron of the color center and the electron makes aquantum transition into an excited state without having been transferredto a conduction band of the crystal.

For the purposes of the present invention, the term “laser light powerdensity” or “laser light intensity” refers to a physical quantitymeasured as an average light energy of the laser beam propagatingthrough the medium per second and divided by the area of laser beamwaist.

For the purposes of the present invention, the term “substantiallyinsensitive to room light” refers to a crystalline material that doesnot change significantly its coloration or concentration of electrons ontraps (concentration of unstable species) under ambient lightconditions.

For the purposes of the present invention, the term “capable of beingused for long-term data storage” refers to a crystalline material thatdoes not change significantly its coloration or concentration ofelectrons on traps (concentration of unstable species) at ambienttemperatures.

For the purposes of the present invention, the term “photo-ionizationcross-section” refers to a parameter having a dimension of cm²/J thatdetermines how much light energy per unit area is required to performphoto-ionization of a color center. The larger the photo-ionizationcross-section means less energy per unit area is required to performionization (recording of the bit).

For the purposes of the present invention, the term “fluorescence yield”refers to the parameter determined as a ratio of the number of photonsemitted by a luminescent material to the number of photons absorbed bythis luminescent material.

For the purposes of the present invention, the term “electron trap”refers to a structural defect in a crystal lattice able to create alocalized electronic state and capable of capturing free electrons froma conduction band of the crystalline material.

For the purposes of the present invention, the term “hole trap” refersto a structural defect in a crystal lattice able to create a localizedelectronic state and capable of capturing free holes from a conductionband of the crystalline material.

For the purposes of the present invention, the term “deep trap” refersto an electron or hole trap having a thermal activation energy largerthan kT, where T is absolute temperature of the crystal and k isBoltzmann's constant.

For the purposes of the present invention, the term “efficient deeptrap” refers to a deep trap which is capable of trapping electrons orholes and which has a sufficient capture cross-section.

For the purposes of the present invention, the term “multilevel opticaldata storage” or “multivalued optical data storage” refers to theability of the data storage system to perform recording and reading ofdata from the same physical location in the medium with a number ofquantized data or signal levels more than two.

For the purposes of the present invention, the term “confocal detection”refers to a method of optically detecting a fluorescence signal in whichthe focal plane inside the medium of the optical read/write head isoptically re-imaged onto a plane which contains an aperture or set ofapertures having size comparable to or smaller than the diffractionlimited spot size of the projected spot of fluorescent data storagemedium.

For the purposes of the present invention, the term “sphericalaberration compensation (SAC)” refers to a technique for compensating orcorrecting the spherical aberration that arises when a high numericalaperture objective (of NA of at least 0.35) is focused to a differentdepth inside the volume of the storage medium. The spherical aberrationcorrection allows to maintain diffraction limited spot size over largerdepth of the medium, preferably larger than 500 microns, via dynamicallychanging the focusing lens properties depending on the depth of thefocus inside the medium achieving diffraction limited spot size at thefocusing depth.

Description

The need for high capacity and high transfer rate computer devices formassive data storage has stimulated a search for new types of media thatare able to exist in two or more stable configurations. By transferringthe storage medium from one configuration into another, one may writeand erase the bit of information, whereas by analyzing the configurationof the medium, the reading of the bit is realized. A large number ofmaterials and techniques have been suggested for data storage and dataprocessing, but only a few of these techniques have found a practicalapplication. Because of the large number of requirements, it isextremely difficult to develop a medium for optical data storagedevices, which preferably meets all these requirements. The followingarticles, the contents and disclosures of which are hereby incorporatedby reference, describe several of the techniques that have beenattempted: International Symposium on Optical Memory and Optical DataStorage 2002, Technical Digest Publication of IEEE, Catalog #02EX552(July 2002); Optical Data Storage 2001, Proceedings of SPIE, Vol. 4342(2001); Optical Data Storage 2000, Proceedings of SPIE, Vol. 4090(2000); International Symposium on Optical Memory and Optical DataStorage 1999, SPIE, Vol. 3864 (1999); Advanced Optical Data Storage:Materials, Systems, and Interfaces to Computers, Proceedings of SPIE,Vol. 3802 (1999); and K. Schwartz, The physics of optical recording,Springer-Verlag, Germany (1993), the entire contents and disclosures ofwhich are hereby incorporated by reference.

Some of the most important requirements that data storage devicespreferably meet are: an ability to repeatedly write, read, and erase thedata (>10⁶ cycles); a high density of bits per unit volume or area(>10¹¹ cm⁻³); a high data transfer rate (>10⁷ bit/s); a minimum accesstime (<10⁻² s); a long lifetime for the storage medium (>10 years) andnon-volatility of the information stored in the storage medium; anenvironmental stability of medium characteristics; safety of the storeddata; and an ability to accomplish data error correction.

Several methods have been attempted to provide storage devices thatmight be able to compete with or replace conventional magnetic inductionmethods to achieve desirable characteristics. Among the methodsattempted have been to use: magneto-optic and electro-optic effects(Pockels effect, Kerr effect, Faraday effect, photorefractive effect,etc.), the photochromic effect in dye polymers and inorganic crystals,and phase transformation in the storage medium at the spot being heatedwith a laser beam. Some of these methods have been successfully realizedin the form of phase change media in the form of CD-RW and DVD-RW, andmagneto-optical WREM discs and drives that are already on the market.Other methods, such as near-field, solid immersion lens recording, andatomic force microscopy are merely contemplated, see Alternative StorageTechnologies Symposium 2001, Monterey Calif., Jun. 26, 2001, the entirecontents and disclosure of which is hereby incorporated by reference.

Most of the conventional techniques mentioned above that use 2D (thinfilm) media are approaching a fundamental limit of storage densitycaused by a minimum achievable focused laser light spot or in the caseof magnetic recording by thermal instabilities of magnetic domain walls(super paramagnetic effect). The most promising way to overcome theselimitations may be to use volumetric (3D-space) recording. Among 3Dtypes of data storage, the types of data storage that have beeninvestigated, most have been in the area of digital holography, seeHolographic data storage, (eds.: H. J. Coufal, D. Psaltis, G.Sincerbox), Springer 2000, p. 488, and volumetric multilayer single-bitrecording, see Confocal and Two Photon Microscopy, Foundations,Applications, and Advances, (ed. Alberto Diaspro) Wiley-Liss, 2002, p.567; the entire contents and disclosures of the above references arehereby incorporated by reference.

Several kinds of materials, such as photopolymers, photochromicmaterials and photorefractive crystals, have been proposed as possiblerecording media with a confocal detection scheme when one bit in thevolume of the medium may be written as a local refractive index changeusing two-photon absorption (2PA) of a high peak-power short pulse laserbeam and the recorded data is read by measuring the change in reflectionof the read laser light, see U.S. Pat. No. 5,289,407 to Strickler, etal.; James H. Strickler, Watt W. Webb, Three-dimensional optical datastorage in refractive media by two-photon point excitation, OpticsLetters, Volume 16, Issue 22, 1780, November 1991; Y. Kawata, H.Ishitobi, S. Kawata, Use of two-photon absorption in a photorefractivecrystal for three-dimensional optical memory, Optics Letters, Volume 23,Issue 10, 756–758, May 1998; A. Toriumi, J. M. Herrmann, S. Kawata,Nondestructive readout of a three-dimensional photochromic opticalmemory with a near-infrared differential phase-contrast microscope,Optics Letters, Volume 22, Issue 8, 555–557, April 1997; M. Ishikawa, Y.Kawata, C. Egami, O. Sugihara, N. Okamoto, M. Tsuchimori, O. Watanabe,Reflection-type confocal readout for multilayered optical memory, OpticsLetters, Volume 23, Issue 22, 1781–1783, November 1998; A. Toriumi, S.Kawata, M. Gu, Reflection confocal microscope readout system forthree-dimensional photochromic optical data storage, Optics Letters,Volume 23, Issue 24, 1924–1926, December 1998; Min Gu, Daniel Day, Useof continuous-wave illumination for two-photon three-dimensional opticalbit data storage in a photo-bleaching polymer, Optics Letters, Volume24, Issue 5, 288–290, March 1999; Yoshimasa Kawata, Takuo Tanaka,Satoshi Kawata. Randomly accessible, multilayered optical memory with aBi₁₂SiO₂₀ crystal, Applied Optics-IP, Volume 35, Issue 26, 5308–5311,September 1996; Daniel Day, Min Gu, Andrew Smallridge, Use of two-photonexcitation for erasable rewritable three-dimensional bit optical datastorage in a photo-refractive polymer, Optics Letters, Volume 24, Issue14, 948–950, July 1999; Y. Shen, J. Swiatkiewicz, D. I. Jakubczyk, F.Xu, P. N. Prasad, R. A. Vaia, B. A Reinhardt, High-Density Optical DataStorage With One-Photon and Two-Photon Near-Field FluorescenceMicroscopy, Applied Optics, Volume 40, No. 6, 938–940, February 2001; T.Wilson, Y. Kawata, S. Kawata, Readout of Three-Dimensional OpticalMemories, Optics Letters, Volume 21, No. 13, 1003–1005, July 1996; H.Ueki, Y. Kawata, S. Kawata, Three-Dimensional Optical Bit-MemoryRecording and Reading With a Photorefractive Crystal: Analysis andExperiment, Applied Optics, Volume 35, No. 14, 2457–2465, May 1996; MinGu, Confocal Readout of Three-Dimensional Data Bits Recorded by thePhotorefractive Effect Under Single-Photon and Two-Photon Excitation,Proceedings of the IEEE, Volume 87, No. 12, 2021–2029, December 1999,the entire contents and disclosures of which are hereby incorporated byreference.

One-bit micro-holograms were suggested as the way to increase bitreflectivity and signal-to-noise and carrier-to-noise ratio (SNR andCNR, respectively), see U.S. Pat. No. 6,322,931 to Cumpston, et al.;U.S. Pat. No. 6,322,933 to Daiber, et al.; H. J. Eichler, P. Kuemmel, S.Orlic, A Wappelt, High-Density Disk Storage by MultiplexedMicroholograms, IEEE Journal of Selected Topics in Quantum Electronics,Volume 4, No. 5, 840–848, September/October 1998; Y. Kawata, M. Nakano,Suk-Chun Lee, Three-Dimensional Optical Data Storage UsingThree-Dimensional Optics, Optical Engineering, Volume 40, No. 10,2247–2254, October 2001, the entire contents and disclosures of whichare hereby incorporated by reference.

Several types of photochromic materials have been proposed for 3Done-bit optical data storage: organic fluorescent dyes dispersed in apolymer matrix, which undergo chemical or structural conformation,diffusion and polymerization as a result of illumination. Two-photonabsorption in fluorescent photopolymers and confocal detection schemeshave also been used, see U.S. Pat. No. 5,325,324 to Rentzepis, et al.;D. A. Parthenopoulos and P. M. Rentzepis, Three-Dimensional OpticalStorage Memory, Science, Vol. 245, pp. 843–845, 1989; Daniel Day, MinGu, Effects of Refractive-Index Mismatch on Three-Dimensional OpticalData-Storage Density in a Two-Photon Bleaching Polymer, AppliedOptics-IP, Volume 37, Issue 26, 6299–6304, September 1998; Mark M. Wang,Sadik C. Esener, Three-Dimensional Optical Data Storage in a FluorescentDye-Doped Photopolymer, Applied Optics, Volume 39, No. 11, 1826–1834,April 2000; E. P. Walker, X. Zheng, F. B. McCormick, H. Zhang, N.-H.Kim, J. Costa, A. S. Dvornikov, Servo Error Signal Generation for2-Photon Recorded Monolithic Multilayer Optical Data Storage, OpticalData Storage 2000, Proceedings of SPIE Vol. 4090, pp. 179–184, 2000; H.Zhang, A. S. Dvornikov, E. P. Walker, N.-H. Kim, F. B. McCormick, SingleBeam Two-Photon-Recorded Monolithic Multi-Layer Optical Disks, OpticalData Storage 2000, Proceedings of SPIE, Vol. 4090, pp. 174–178, 2000; Y.Zhang, T. D. Milster, J. Butz, W. Bletcher, K. J. Erwin, E. Walker,Signal, Cross Talk and Signal to Noise Ratio in Bit-Wise VolumetricOptical Data Storage, Technical Digest of Joint International Symposiumon Optical Memory and Optical Data Storage, IEEE Catalog, No. 02EX552,pp. 246–248, 2002; E. P. Walker, W. Feng, Y. Zhang, H. Zhang, F. B.McCormick, S. Esener, 3-D Parallel Readout in a 3-D Multilayer OpticalData Storage System, Technical Digest of Joint International Symposiumon Optical Memory and Optical Data Storage, IEEE Catalog, No. 02EX552,pp. 147–149, 2002; and Ingolf Sander (Constalation 3D, Inc.) Fluorescentmultilayer technology, In: Alternative Storage Technologies Symposium2001, Monterey Calif., Jun. 26, 2001, the entire contents anddisclosures of which are hereby incorporated by reference.

Using luminescent materials as data storage media is especiallyattractive because of their ability to realize multilevel (ormultivalued) optical data storage. Luminescent response is proportionalto the product of the energy deposited in the medium during “writing”and “reading”. If the concentration of defects undergoing electronictransition in the volume corresponding to one bit of information islarge enough, then that element of the light-sensitive medium may beused in a “gray scale” mode and the optical data storage system may beused as a multilevel (or multivalued) data storage system. The potentialstorage capacity is increased proportionally to the number of datalevels reliably achieved. The total linearity of luminescent responsemay stretch over several orders of magnitude. Different logical statesof the medium may be represented by different intensities of theluminescent signal and digitized using thresholding electronic circuits.In practice, 10 levels of fluorescent intensity may be achieved bychanging the energy or the time duration of the laser “writing” beam. Anincreased density of data storage is one of the main potentialadvantages of the luminescent techniques of the present invention.

Similar approaches to writing to and reading from a data storage mediumhave been demonstrated in silver-doped photoluminescent glasses used inradiation dosimetry, see B. Lommler, E. Pitt, A. Scharmann, Opticalcreation of radiophotoluminescence centers in dosimeter glass bytwo-photon absorption, Radiat. Prot. Dosim. Vol. 65, No. 1–4, pp.101–104 (1996), the entire contents and disclosure of which is herebyincorporated by reference. Two-photon UV excitation (“writing”) producesa photoluminescence signal that may be repeatedly “read” with the samelaser, but at lower power, without measurable erasure of information.However, how such data may be “erased” without heating the medium is notclear. Complications may also be caused by the long-term process ofdiffusion and luminescent center transformation that lead to a“build-up” of luminescent signal.

A promising way to overcome the limitations of conventional data storagesystems is to use volumetric or 3D recording. Among 3D-technologies,multilayer one-bit recording with two-photon absorption (2PA) has someclear advantages. The probability of two-photon absorption isproportional to a square of laser light intensity. Two-photon absorptionallows one to perform photo-ionization or photo-transformation of aphotosensitive medium only in the vicinity of a tightly focused laserbeam without affecting the surrounding volume of the material. The sizeof one three-dimensional bit or voxel written using 2PA may be made assmall as 1×1×3 μm. Extremely high storage density of up to 10 Tbits/in³is expected.

An apparatus built according to the present invention is illustrated byFIG. 1, described in more detail below. Two lasers based on blue laserdiodes are used to write and to read the data from a storage medium.Two-photon absorption from a more powerful CW laser having a shorterwavelength is used for recording the data. High NA of the objective lensallows for the high intensity of the laser light needed for 2PA to beachieved and for the formation of a diffracted limited recorded bitsize. One-photon-induced fluorescence induced by low energy and longerwavelength laser and a confocal detection scheme may be used for readingthe data. Confocal detection allows one to significantly reducecross-talk between adjacent bits, tracks, and layers with the purpose ofachieving a desirable signal-to-noise ratio (SNR).

An important objective of the present invention is to provide a methodand apparatus that performs writing operation using high probability of2PA and makes the reading operation non-destructive by decreasing theprobability of 2PA during the readout. At the same time the method andapparatus according to the present invention use 1PA and laser-inducedfluorescence during readout at a highest possible level needed toachieve acceptable SNR and high data transfer rate.

Most of the problems with the various storage media described above havebeen related to inadequate material properties. Most of thephotopolymers suggested for one-bit or holographic data storage showhigh sensitivity but exhibit strong dimensional shrinkage. Most of thephoto-sensitive polymers may be used only as WORM media (write once,read many times), whereas rewritable photopolymers are still unstableand show significant fatigue when write-read cycles are repeated manytimes. Even write-once fluorescent photopolymers show strong reductionof fluorescent output signals when read repeatedly. An additionalobstacle for most suggested photopolymers and photorefractive crystalstested for volumetric one-bit recording is a necessity of using afemto-second high peak power Ti-sapphire laser to achieve efficienttwo-photon absorption, because a Ti-sapphire laser is big, expensive andat the time are suitable mostly for laboratory demonstration.

Therefore, utilization of an efficient and stable inorganic photochromicfluorescent material for one-bit optical recording and reading is anobjective of the present invention.

The low thermal energy depth of the traps responsible for capturingelectrons produced by ionization with the laser light during writingcauses thermally stimulated release of electrons from traps at ambienttemperatures and fading of the stored data. This is generally notacceptable, especially for multivalued storage, which requires precisedigitization of the analog luminescent signal. Furthermore, the chemicalinstability of some luminescent materials and their sensitivity tooxidization and humidity when contacted with air require the use ofprotective layers. In some organic fluorescent materials, dimensionalstability is a significant problem because of material shrinkage as aresult of photochemical transformation and polymerization.

Electronic transitions in solids caused by light excitation withrelaxation times on the order of 10⁻¹² to 10⁻⁹ s are fundamentally veryfast and are considered among the most promising quantum systems formassive optical data storage. Luminescence decay time after a pulse oflaser stimulation determines the time needed for retrieval of each bitof information and the maximum achievable data transfer rate.

To achieve more stable and reliable data storage and optical processing,one should use chemically, mechanically and thermally stable luminescentmaterials with deep traps and luminescent centers. To produce such deepcenters, one needs to use a wide gap dielectric. Furthermore, these“thermally” and “optically” deep traps require a shorter wavelength oflaser light for excitation (“writing”), stimulation (“reading”) andrestoration (“erasing”). For optical recording, the minimum light spotdiameter is equal to: d≈0.5 λ/NA, where NA is the numerical aperture ofthe optical head. Therefore, blue and UV lasers have a clear advantageagainst IR lasers for achieving higher storage densities. The latestdevelopments in blue and UV solid state lasers, based onheterostructures of wide gap semiconductors like GaA1N create a realpossibility for use of materials with wide energy gaps.

The low energy depth of the traps responsible for the accumulation ofthe charge carriers leads to the thermally stimulated release ofelectrons at ambient temperatures and to fading of the stored data. Thisis not acceptable, especially for multilevel data storage, whichrequires precise digitizing of the analog luminescent signal.Furthermore, the chemical instability and sensitivity to oxidization andhumidity when contacted with air require the use of protection layers.

Corundum or sapphire (α-Al₂O₃) is an important technological material inmany optical and electronic applications. It is used as a host materialfor solid-state lasers, as optical windows, as a substrate material insemiconductor epitaxial growth and, more recently, as a radiationdetector see M. S. Akselrod, V. S. Kortov, D. J. Kravetsky, V. I.Gotlib, Highly Sensitive Thermoluminescent Anion-Defective α-Al2O3:CSingle Crystal Detectors, Radiat. Prot. Dosim., Vol. 32(1), pp. 15–20(1990). In spite of excellent dosimetric properties of carbon-dopedAl₂O₃ with oxygen vacancies, the luminescent centers in this material(F-centers) have a very long luminescence lifetime (35 ms). However,α-Al₂O₃ is unacceptable for fluorescent one-bit recording applicationsrequiring a high data transfer rate. Known Al₂O₃ materials also do nothave absorption bands that may undergo photochromic transitions suitablefor volumetric data storage applications.

With respect to the new Al₂O₃ crystalline material described in U.S.application Ser. No. 10/309,021, filed Dec. 4, 2002, and U.S.application Ser. No. 10/309,179, filed Dec. 4, 2002, the entire contentsand disclosures of which are hereby incorporated by reference, importantfeatures of this material utilized in the present invention are theelectronic and optical properties of a storage phosphor and its defectstructure. The Al₂O₃:C,Mg crystalline material has color centersabsorbing light, stable traps of electrons and holes and its luminescentcenters have a short luminescence lifetime.

A simplified scheme illustrating how the crystalline fluorescentmaterial may be used in an optical data recording and retrieving driveis illustrated by FIG. 1. In scheme 100, a 405 nm “write” laser beam 102and 440 nm “read” laser beam 104 are produced by two diode lasers 106and 108, respectively. Laser beams 102 and 104 are directed on a singlecrystalline disk 110 made of Al₂O₃:C,Mg through a flipping mirror 112,dichroic mirror 114, and a high NA focusing objective lens 116. Disk 110spins as shown by arrow 118 or moves by a 3D translation stagerepresented by arrows 119. An optical pick-up head that combinesobjective lens 116, dichroic mirror 114, focusing lens 120, a confocalpinhole 122, and a photodetector 124 slides along the radius of the disk110. Selection of the focal depth of the bit 126, a certain data layerwithin 3D volume of the disk and correction of spherical aberrations areperformed by moving an additional optical component of an opticalpick-up head (not shown). A photodetector 124 is used to monitorlaser-induced fluorescence 128 during writing and reading. A greenfluorescence 126 is collected by objective lens 116, reflected by thedichroic mirror 114, focused by lens 120 on confocal pinhole 122, anddetected by a photon counter or a digital oscilloscope 130 interfacedwith a computer 132.

In the scheme shown in FIG. 1, two-photon absorption during a “write”operation allows for very tight localization of the photo-ionizationprocess in the focal spot of the laser beam and 3D confinement of thewritten bit (green fluorescence 126). One-photon excitation of thefluorescent light from the medium using “read” laser light of a longerwavelength and a confocal fluorescence detection scheme allows one toperform non-destructive reading of bits multiple times.

The method described in the present invention is made possible due tothe unique optical properties of Al₂O₃:C,Mg. The primary informationstorage process in Al₂O₃ is photoionization, followed by the subsequentcapture of the excited electronic charge by trapping centers. Thus, forthe efficient storage of information, it is necessary that Al₂O₃crystals contain both light absorbing color centers and defects capableof trapping electrons. High quantum yield of fluorescence is alsorequired for one-bit confocal recording. According to a preferredembodiment of the present invention volumetric recording of bits isperformed using 2PA, whereas non-destructive reading operation utilizes1PA and fast laser induced fluorescence.

The Al₂O₃ crystalline materials produced according to a method describedin U.S. application Ser. No. 10/309,021, filed Dec. 4, 2002, and U.S.application Ser. No. 10/309,179, filed Dec. 4, 2002, the entire contentsand disclosures of which are hereby incorporated by reference, andutilized in the present invention include several types of oxygenvacancy defects and are doped with carbon and magnesium impurities andcan be grown by those skilled in the art using any conventional crystalgrowth method (for example using the Czochralski method). Thecrystalline material utilized in the present invention is characterizedby several optical absorption (OA) bands: at 205, 230, 255, 335, 435,and 630 nm (FIG. 2). The blue absorption band at 435 nm is responsiblefor the visible green coloration of the crystal. One important featureof the new aluminum oxide material is a high concentration of single anddouble oxygen vacancies in the form of neutral F-centers as well as F⁺and F₂ ²⁺ centers, charge-compensated by the nearby Mg-impurity atoms.

An F⁺-center is an oxygen vacancy with one electron charge-compensatedby one Mg²⁺-ion and is denoted as an F⁺(Mg)-center. This center ischaracterized by at least two absorption bands at 230 and 255 nm (FIG.2) and has a luminescence band at 330 nm with a lifetime of less than 5ns. A cluster of two of these defects forms an aggregate vacancy defectcomposed of two F⁺-centers and two Mg-impurity atoms. This aggregatedefect with two localized electrons, denoted here as F₂ ²⁺(2Mg), isfavorable for optical data storage. It is responsible for a blueabsorption-excitation band at 435 nm (FIG. 3), produces a greenfluorescence band at 520 nm, and has a short lifetime equal to 9±3 ns(FIG. 4).

Exposure of an Al₂O₃:C,Mg crystal having oxygen vacancy defects to highintensity laser light of appropriate wavelength results in conversion ofthe same structural defect from one charged state into another. Forexample, F₂ ²⁺(2Mg)-centers are converted into F₂ ⁺(2Mg)-centers with430 nm illumination (FIG. 5) and may be converted back with 335 nmpulsed laser light. After photochromic transition induced by blue laserlight, Al₂O₃:C,Mg crystals exhibit an absorption/excitation band at 335nm (FIGS. 5 and 6), a broad fluorescent emission at 750 nm (FIG. 6) withrelatively fast decay time equal to 80±10 ns (FIG. 7).

In a preferred embodiment of the present invention a two-photonabsorption (2PA) process is utilized for recording the information inthe volume of Al₂O₃:C,Mg. Usually 2PA is considered as a process ofsimultaneous absorption by the luminescence center of two photons. Thesum of energies of these two photons is preferably enough to performexcitation of the luminescent center whereas energy of only one photonis not sufficient for the excitation transition. 2PA in that case isperformed through the virtual (non-existing) quantum energy state ofdefect and the probability of it is very low. To perform 2PA, femto- orpicosecond laser pulses with a power density on the order of 100 MW/cm²are required. A very important trait of Al₂O₃:C,Mg, which enablesso-called “sequential” 2PA, is that defects absorbing the laser lightand producing the fluorescence have an excited state located deep in theenergy gap. The lifetime of this excited state is sufficiently long tosignificantly increase the probability of a second photon absorptionneeded for photo-ionization and data recording. At the same time thelifetime of the excited state for this color center is short enough toallow fast reading of a fluorescent signal with a high data transferrate.

The evidence of preferred two-photon absorption in aggregate oxygenvacancy defects is provided by quadratic dependence of thephoto-ionization cross-section of these centers versus laser lightintensity (see FIG. 8). Photo-ionization cross-section for the 2PAprocess is inversely proportional to the decay constant and directlyproportional to the product of the absorption cross-sections of theground and excited states. Wavelength dependence of photo-ionizationcross-section of F₂ ²⁺(2Mg) centers is shifted to a shorter wavelengthin comparison with the 1PA band (FIGS. 3 and 9) and is anotherindication of the 2PA process in Al₂O₃:C,Mg crystals.

Erasing of written data and restoration of original optical absorption(coloration) and fluorescence of the storage medium according to thepresent invention can be achieved optically or thermally. Al₂O₃:C,Mgcrystalline material includes deep traps of charge. These deep traps ofcharge have a delocalization temperature about 600° C. to 700° C. andhave a concentration about 10¹³ to 10¹⁷ cm⁻³. The delocalizationtemperature of these deep traps was found from the optical absorptionexperiment (see FIGS. 10A, 10B, 10C and 10D) with step annealing of theAl₂O₃:C,Mg crystal after it was illuminated with a 430 nm pulsed laserlight that is equivalent to a “write” operation in the optical datastorage system. Optical absorption bands of F⁺-centers at 255 nm and F₂²⁺(2Mg)-centers at 435 nm increase their intensities and restore theiroriginal intensity in the temperature region between 600 and 700° C. Theopposite trend may be seen in the same temperature range for 335 nm bandof F₂ ⁺(2Mg)-centers and 630 nm band of F₂ ³⁺(2Mg)-centers indicatingthat these centers convert into F₂ ²⁺(2Mg)-centers during annealing. 630nm absorption band of F₂ ³⁺(2Mg)-centers appears only after “write”operation with the pulsed blue laser light and is not visible in theabsorption spectrum of the fresh Al₂O₃:C,Mg crystal shown in FIG. 2.

Optical erasure of recorded bits by reverse photochromic transformationand restoration of the 435 nm absorption band and 520 nm fluorescencecan be achieved according to another preferred embodiment of the presentinvention using sequential illumination of the Al₂O₃:C,Mg crystal with205±30 and 335±30 nm laser light corresponding to absorption bands of Fand F₂ ⁺(2Mg) centers. Illumination with the light at 205±30 nmcorresponding to an F-absorption band performs photo-ionization ofF-centers and generates free electrons. These free electrons can becaptured by the defects ionized during recording. In particular theseelectrons are captured by deep hole traps and F₂ ²⁺(2Mg) and F₂³⁺(2Mg)-centers. The goal of one of the preferred embodiments of thepresent invention is to convert as many double oxygen vacancy defects aspossible into a F₂ ⁺(2Mg) charge state. These defects are characterizedby an intensive absorption band in the region of 335 nm. Ionization of Fcenters can be performed by either coherent laser light or incoherentlight, produced for example by deuterium of xenon lamps. Subsequentillumination of the crystal with the high power density laser lighthaving 335±30 nm performs 2PA on F₂ ⁺(2Mg) centers, converts them intoF₂ ²⁺(2Mg)-centers. The described optical procedure restores a 435 nmabsorption band and 520 nm fluorescence characteristic to these defectsand restores original green coloration of the Al₂O₃ :C,Mg crystal.

Preferred electronic processes during “write” and “read” operation inthe utilized Al₂O₃:C,Mg material of the present invention are explainedusing a band diagram as in FIG. 11. A preferred doped Al₂O₃ material ofthe present invention for use as a data storage medium may be formed tocontain a high concentration of trapping sites and fluorescent centerswith precisely desirable characteristics. Data storage media generallyexist in at least two stable physical states assigned correspondingly to“0” and “1” states. An initial configuration (logical 0 state) ofas-received or erased Al₂O₃ medium has a high concentration of F₂²⁺(2Mg)-centers, characterized by an intensive absorption band in theregion of 435±5 nm.

By illumination with the writing laser light (“write” beam”) of theappropriate photon energy hv₁ (or wavelength λ₁) and intensity, which ishigh enough to ionize the above described crystal defects, one mayproduce free electrons to be trapped in pre-existing electronic defects.The traps in Al₂O₃:C,Mg are deep enough to keep the charge carriers fora long time at ambient temperature without being thermally released.This second state of a quantum system is now in a metastable “charged”configuration (logical “1” state). To “read” the state of the medium,the stimulation light of the same as writing light or another photonenergy hv₂ (or wavelength λ₂) is applied and fluorescent photon ofenergy hv₃ (or wavelength λ₃) is detected. In case of fluorescentone-bit recording, a written bit produces reduced fluorescence intensitywhereas an unwritten spot produces original intensive fluorescence.

Electronic defects in wide gap dielectrics like Al₂O₃ are characterizedby the energy levels of their ground and excited states. If the excitedstate of the electronic defect is located close or within the conductionband, the defect may be ionized by one-photon absorption. A differentsituation takes place when the excited state is located deep within theenergy gap of the crystal. Absorption of one photon corresponding to theenergy transition between ground and excited states of the electronicdefect results in a localized transition followed by non-radiative andradiative decay (fluorescence). This one-photon absorption process isnondestructive and may be used for reading information.

To remove an electron from such a deep defect and to change theluminescent properties of the particular defect in the crystal,simultaneous absorption of two photons is used in the present invention.The first absorbed photon excites the electron of the above describedelectronic defect into its excited state, whereas the second photontransfers the electron from the above described excited state within theenergy gap into the conduction band. An electron in the conduction bandis now delocalized (removed from its original defect site) and may betrapped by another defect.

Writing the data may be performed (see FIG. 11) using two-photonabsorption of 435±40 nm blue laser light by the F₂ ²⁺(2Mg)-centersdescribed above. The first photon excites one of the two electronslocalized on the center to its excited state while the second photonperforms the second transition between the excited state and theconduction band, thus performing photo-ionization of the center. Thefinal stage of the writing process is localization (or trapping) of theabove described photoelectron by another defect, namely by another F₂²⁺(2Mg)-center, or by F⁺(Mg)-center or by carbon impurity. The result ofthese photochromic transformations is (a) creation of another chargedstate of the aggregate defect, F₂ ⁺(2Mg)-center, having three localizedelectrons and characterized by a UV absorption band at 335 nm, or (b)creation of a neutral F-center with a UV absorption band at 205 nm, or acarbon related trap responsible for 190° C. TL peak. All three processesresult in formation of optically deep and thermally stable electronicstates and may be used in a preferred embodiment of the presentinvention for long-term data storage. The first process (a) has a higherprobability that was determined from the efficiency of photo-conversionof optical absorption bands. As a result of photo-ionization, an F₂²⁺(Mg)-center converts into F₂ ³⁺(Mg) that has an absorption band at 620nm and the released electron is trapped by another F-₂ ²⁺(Mg)-centerconverting it into an F₂ ⁺(Mg)-center having three localized electronsand characterized by an absorption band at 335 nm and an emission bandat 750 nm.

The present invention provides two types of fluorescent processes forreading data (see FIG. 11). The Type 1 or “negative” process involvesstimulation of original green fluorescence of F₂ ²⁺(2Mg)-centers usingblue laser light excitation at 435±40 nm. The intensity of thisexcitation is preferably significantly reduced to avoid two-photonabsorption, but sufficient to generate green fluorescence enough for thereliable detection of information. Small volumes of Al₂O₃ crystal(voxels) subjected to two-photon ionization during writing show reducedor no fluorescence, whereas the unwritten voxels show high intensity ofgreen fluorescence.

A type 2 readout process of the present invention, the so-called,“positive” readout process, involves using laser excitation at 335±30 nmto stimulate the fluorescence of F₂ ⁺(2Mg)-centers created duringrecording. The intensity of this excitation also is preferablysignificantly reduced to avoid two-photon absorption. The intensity offluorescence of F₂ ⁺(2Mg)-centers in the region of 750 nm having an 80ns lifetime is used as a measure of data during a “readout” process forbinary or multilevel data storage.

Writing the data on the aluminum oxide medium of the present inventionusing 2PA and reading of data from the same medium may be performed bymeans of laser induced fluorescence (LIF) and is preferably achievedthrough electron motion and electronic transitions. No phasetransformation or other structural changes happen during “write” or“read” operations. This makes the data recording and retrievingprocesses extremely fast and reproducible.

A process of writing data according to a preferred embodiment of thepresent invention will now be described. First the data storage mediumin the form of a Mg-doped anion-deficient Al₂O₃ single crystal is movedto a desired position with respect to the diffraction limited laser“write” beam, focused on a predetermined depth of the medium by means ofmechanical motion of the medium and/or the adjustable components of theoptical head. Spherical aberration compensation of the focused laserbeam is also performed at this stage by means of mechanical motion ofthe optical component or by electro-optical component based for exampleon liquid crystal phase shifter.

Then, the data storage medium is illuminated with the above-describedfocused beam of writing laser light having wavelength λ₁ for the periodof time equal to a write time t₁. The above described “write” wavelengthλ₁ is in the range of 370–490 nm with a more preferred wavelength equalto 390 nm. The sequential two-photon absorption process described in thepresent invention can be achieved at laser intensity of higher than 1kW/cm². The “write” time is in the range of 0.1 ps to 1 ms with a morepreferred time equal to 10 ns. The result of the writing operation isionization and photo-conversion of F₂ ²⁺(2Mg)-centers into F₂³⁺(2Mg)-centers:F₂ ²⁺(2Mg)+2hv ₁=F₂ ³⁺(2Mg)+e ⁻

Electrons released from F₂ ²⁺(2Mg)-centers as a result of thephoto-ionization process are captured by deep traps and other nearby F₂²⁺(2Mg)- and F⁺-centers:F₂ ²⁺(2Mg)+e ⁻=F₂ ⁺(2Mg)F⁺(Mg)+e ⁻=F(Mg)

The above-described deep trapping sites are able to store informationalmost indefinitely.

The present invention provides three preferred modes of reading datausing a confocal laser induced fluorescent detection scheme. The firstmode reads the data with the same laser beam of wavelength λ₁ used for“writing”, but with significantly reduced intensity and time ofillumination to avoid two-photon absorption and erasure of stored data.In the second mode of operation, the reading laser beam has a wavelengthλ₂ longer than λ₁, but it is still within the absorption band of F₂²⁺(2Mg)-centers. For example, wavelength λ₂ is selected to be 460 nm. Alonger wavelength further reduces the probability of 2PA and allows forhigher laser light intensity for excitation of fluorescence and providesbetter signal-to-noise ratio (SNR). These two modes of “read” operationutilize a fluorescent emission band of F₂ ²⁺(2Mg)-centers in the regionof 520 nm (FIG. 3). Lifetime of this fluorescence is 9±2 ns (FIG. 4) andis fast enough to achieve a 100 Mbit/s data transfer rate. A strongfluorescence signal corresponding to a 0 binary state indicates that no“write” operation was performed on the particular bit.

These two first modes of reading may be referred to as “negative” typesof operation. The third mode of reading operation utilizes fluorescenceof F₂ ⁺(2Mg)-centers (three electrons occupying the aggregate defect)created as a result of trapping the electron by the F₂ ²⁺(2Mg)-centersduring writing operation:F₂ ²⁺(2Mg)+e ⁻=F₂ ⁺(2Mg)

F₂ ⁺(2Mg)-centers may be excited in their absorption band in the regionof 335 nm or in F⁺-center absorption band at 255 nm. Emission of thesecenters is in the infrared region and is in the region of 750 nm (seeFIG. 6 for details of excitation and emission spectra). The lifetime ofthe 750 nm emission is 80±10 ns (see FIG. 7) and is short enough for adata transfer rate up to 10 Mb/s.

A preferred reading operation of the present invention will now bedescribed. First, the above described data storage medium is moved to adesired position with respect to a focused “read” laser beam. Theabove-described read laser beam has a wavelength 2 is in the range of390–500 nm with a more preferred wavelength equal to 460 nm. Then, theabove described data storage medium is illuminated with the abovedescribed focused beam of “read” light for the period of time equal to a“read” time t₂. The above described “read” time t₂ is in the range of0.1 ns to 1 ms with the more preferred time in the range between 2 and15 ns most preferred time equal to 5 ns. The laser-induced fluorescenceproduced by the Al₂O₃ data storage medium is then measured using aphotodetector. The above-described LIF is the “data” light at the thirdwavelength 23 in the region of 750 nm and is in the range from 620 nm to880 nm. The above-described fluorescent signal is then processed toobtain the value of the stored data.

In another preferred embodiment of the present invention fluorescenceintensity from the recorded bit is inversely proportional to the amountof energy (or number of “write” laser pulses) delivered to the bitduring the recording stage (FIGS. 17 and 18). It can be digitized forbinary or multilevel types of data and thus can be used for furtherincrease of density of data storage.

The present invention also allows parallel processing of multiple markson the storage medium for further increase of “write” or “read” rate anddata storage density. Parallel processing is one of the main advantagesof optical data recording. One may use one-dimensional ortwo-dimensional arrays of lasers and photo-detectors (CCD chips or CMOSdetectors).

The storage medium of the present invention also provides thermal,temporal and environmental stability of the medium and stored data. Thecommon problem for fluorescent and photorefractive data storage mediumis the thermal instability and result in thermal erasure of storedinformation. Al₂O₃ doped with carbon and magnesium exhibits extremelygood thermal and temporal stability of information stored as electronstrapped on localized states formed by oxygen vacancy defects in thecrystal structure. Lifetime of the charge carriers on traps depends onstorage temperature. The higher the temperature, the smaller thelifetime. The deeper the traps—the longer the storage time. Most of thetrapped electrons are associated with a 650° C. trap that has extremelyhigh thermal and optical depth. Al₂O₃ crystals are very mechanically,chemically and optically stable and do not show degradation ofperformance for years. It was also shown that the recorded data is noterased by conventional room light illumination and the medium does notrequire light protection.

In another preferred embodiment of the present invention, the utilizedmethod of optical data storage is capable of being used for long-termdata storage.

In another preferred embodiment of the present invention, the utilizedmethod of data recording requires laser energy of as small as 15 nJ perbit of information stored in the material.

In another preferred embodiment of the present invention, the utilizedAl₂O₃:C,Mg crystalline material is substantially insensitive to roomlight in both written and erased states.

Compared with known technologies for optical data storage, the presentinvention provides several advantages. Utilization of fundamentally veryfast electronic processes vs. comparatively slow phase changetransitions and photo-induced polymerization for well known techniquesprovides a data transfer rate for one channel of up to 100 Mb/s. Highdata storage density is achieved due to 3D capability of the proposedmaterials and confocal detection schemes restricted only by the bluelaser light diffraction limit and NA of the optical head. Multiple datalayers may be accessed in the bulk of the medium during writing andreading operations using two-photon absorption techniques and confocaldetection schemes. Non-volatile reading is achieved using one-photonexcitation of fluorescent centers causing no degradation of storedinformation. Multilevel (multivalue) data storage may further increasedata storage density due to linearity of luminescent response. Lowaverage laser light intensities required for “writing” and “reading” ofinformation (mW range) allows one to preferably use compact blue laserdiodes. Well-established and efficient crystal growth technologyproduces Al₂O₃ crystals of high optical quality.

The present invention will now be described by way of example. Theexample experiments described below are meant to be illustrative of thematerial and procedure described above and should not be considered tobe definitive descriptions of the invention.

EXAMPLE I

An optical data storage apparatus of the type illustrated in FIG. 1 wasused for demonstrating the methods of the present invention. Both“write” and “read” laser beams were produced by respective semiconductorlasers built using Nichia laser diodes. Two types of writing lasers weretested: CW modulated laser from Power Technology, producing 18 mW ofpower at 405 nm and a PicoQuant pulsed laser, generating 1.5 mW at 411nm (20 MHz, 60 ps pulse duration, 400 mW of peak power). Power of the“read” laser (440 nm, 3 mW) from Power technology was controlled usingneutral density filters. The two laser beams were directed on theAl₂O₃:C,Mg crystalline storage media through a flipping mirror, dichroicmirror and a Nikon CFI PLAN FLUOR (0.85 NA, 60X) objective lens. Thisinfinity conjugate objective lens has an optical component for manualspherical aberration compensation.

A single crystal disk of Al₂O₃:C,Mg was attached to the Polytec PIcombined 3D stepper-piezo translation stage having 10 nm resolution.Selection of the focal depth or a certain data layer within 3D volume ofthe disk and correction of spherical aberrations was performed by movingthe crystal 110 in the Z-direction and by rotating the correcting collaron the objective lens. The laser-induced fluorescence was collected bythe objective lens and was reflected by the dichroic mirror through afocusing lens on the confocal pinhole. Long-pass yellow glass filterOG-515 134 was installed in front of the PMT to reject the residual bluelaser light. Fluorescence detected by the photomultiplier tube wasprocessed either by the digital oscilloscope or by the Stanford ResearchSR430 multichannel photon counter interfaced with a computer.

Recording of diffraction-limited bits at the different depth of Al₂O₃crystal requires careful spherical aberration compensation (SAC). SAC ofthe optical system was calibrated using knife-edge technique with aspecially made sapphire wedge having a thickness variation from 50 to300 μm. After the proper focal plane position and optimum SAC weredetermined, both “write” and “read” laser beam profiles were measured.The diameter of a focused laser beam at 1/e² is equal to 0.55±0.05 μmfor a 100 to 240 μm sapphire depth range.

EXAMPLE II

An Al₂O₃:C.Mg crystal plate 1.8 mm thick was cut from a crystal boule 45mm in diameter. It was polished on both sides and installed in the teststand, described in the Example I, perpendicular to the optical axis ofthe objective lens with the crystal optical caxis parallel to thepolarization of the laser beam. The concentration of color centersresponsible for the blue absorption band at 435 nm and greenluminescence at 520 nm was estimated to be 17,000 centers per cubicmicron.

The test was performed in the following sequence. The “write” operationwas done using either modulated CW or pulsed laser diodes controlled bythe computer interface board. Both types of lasers gave similar results,but the pulsed laser requires less energy per bit and produces betterspatial resolution. The crystal medium was moved in the XY plane in stepincrements using piezo-actuators. During readout the crystal medium istranslated in a ramp mode in X direction and with the step-increments inY and Z directions. The CW laser beam (15 μW, 440 nm) excited green (520nm) fluorescence from the crystal medium.

A high density data storage process for Al₂O₃:C:Mg utilizing two-photonabsorption during one-bit recording and confocal fluorescent detectionscheme for reading is illustrated by the image of FIG. 12. The bit imagein fluorescent contrast was obtained using the method described in thepresent invention and was performed using an apparatus described abovein the Example I and depicted in FIG. 1 in the following sequence. The“write” operation was done with a 405 nm diode laser beam at full powerand the laser pulse duration was controlled with TTL pulses from thecomputer interface board. Decay of the fluorescent signal during writingoperation was detected by the PMT and the oscilloscope and it was anindication of the successful writing. During reading operations, a CWlow power blue diode laser (0.1 mW, 440 nm) modulated by anothersequence of TTL pulses from a computer was used and green fluorescenceseparated by the dichroic mirror and the confocal pinhole was detectedby the PMT and the photon counter.

Matrix of 3 by 3 bits spaced 5 μm apart was recorded and read as animage in fluorescent contrast (see FIG. 12). Nine bits were written with405 nm laser light and with recording energy of just 25 nJ per bit. The“read” operation was performed by scanning of the recorded area of thecrystal storage medium with the modulated CW laser diode having awavelength at 440 nm that is longer than that of the “write beam” toprevent erasure of the information. To obtain the image of the writtenbits, scanning of the storage medium was performed with piezo-actuated3D stage from Polytec PI. The single photon pulses of the fluorescentsignal were detected using PMT and a Stanford Research SR430multichannel photon counter interfaced with a personal computer.Scanning of the crystal was performed at 0.2 μm increments and with a153 μm/s scan rate. The modulation depth of the recorded bits was about40% and a full width at half maximum for a single bit was equal to about1 μm.

EXAMPLE III

High density recording utilizing the method and the apparatus describedin Example I was demonstrated. A 100×100 bit image with 1 μm incrementsin the X and Y directions (FIG. 13) was written using the 3Dpiezo-actuator. Each bit was written with 15 nJ of energy produced bypulsed PicoQuant diode laser (1000 pulses per bit). Reading of the bitpattern in fluorescent contrast was performed with a modulated CW-laserbeam (440 nm, 15 μW) by scanning a raster with 200 mn between lines. Animage having 500×500 pixels was obtained. Spatial profile of severalbits spaced 1 μm apart is shown in FIG. 14 and demonstrates a 12%modulation depth.

EXAMPLE IV

Single bits written using 2PA in the volume of Al₂O₃:C,Mg have differentdimensions in lateral and axial direction with respect to the laser beampropagation direction. Theoretically axial size of the bit should be 3to 5 times bigger than lateral bit size. To determine the size of thebits written in Al₂O₃:C,Mg in the axial direction (XZ plane), bits ofdata were written using a step increment motion of the 3D translationstage in XY plane, as described above in Example III, and then the imageof the bits in fluorescent contrast was obtained by scanning the crystalin the XZ plane (FIG. 15). In yet another test, three layers of recordedbits were obtained at an average depth of 100 μm inside the crystal witha 17 μm separation between layers (FIG. 16). Each layer of bits wasrecorded and read with manual spherical aberration compensation (SAC) ofthe objective lens, which explains some distortion of the fluorescentimage. Automated SAC should allow one to obtain up to 100 layers of databits in the volume of the crystal having 2 mm in thickness with 2Dequivalent of 10 Gbit/cm² of data storage density or close to 1 Tbit ofdata per disk having surface area equal to 100 cm².

EXAMPLE V

FIGS. 17 and 18 illustrate the test of multilevel data storage utilizingthe Al₂O₃:C,Mg optical storage medium. The multilevel recording is basedon the inverse proportionality between the fluorescent intensity of thewritten bits and the number of writing pulses. Ten bits were written inthe Al₂O₃:C,Mg crystal with incremental number of “writing” laserpulses. Modulation depth of the produced bits is a nonlinear function ofthe number of laser pulses but nevertheless can be digitized ontoseveral data values and even further increase the density of datastorage utilizing the method and the medium of the present invention.

EXAMPLE VI

The optical properties of Al₂O₃:C,Mg crystals utilized in the tests ofthe present invention now will be described. Al₂O₃:C,Mg crystals in theshape of a boule having a 45 mm diameter were obtained. Crystals werethen cut in to 1.8 mm thick disks and polished on both sides to obtainoptical quality surfaces. Optical absorption spectra of the Al₂O₃:C,Mgcrystalline material utilized in the present invention and of a knownAl₂O₃:C crystal were obtained using Shimadzu UV-2104PC spectrophotometerand are shown in FIG. 2. The intensity of F⁺-bands at 230 and 255 nm issignificantly higher in Mg-doped crystals. That indicates higherconcentration of F⁺-centers, charge compensated by the Mg²⁺-ions. A blueabsorption band at 435 nm indicates the creation of aggregate F₂ ²⁺(2Mg)defects used in the present invention. The grown crystal had 30 cm⁻¹ ofabsorption in the F-center band at 205 nm and an absorption coefficientof 10 cm⁻¹ in the F⁺-centers absorption band at 255 nm and 1.2 cm⁻¹ ofabsorption at 435 nm corresponding to absorption of F₂ ²⁺(Mg)-center(see FIG. 2). All absorption coefficients are presented aftersubtraction of the background pedestal. According to Smacula's formula,an absorption coefficient may be converted into a concentration ofF-centers equal to 8.6·10¹⁷ cm⁻³ and concentration of F⁺-centers equalto 2.6·10¹⁷ cm⁻³ and 1.7·10¹⁶ cm⁻³ of F₂ ²+(Mg)-centers. The laternumber indicates that there are 17,000 fluorescent centers per cubicmicron of a storage medium.

EXAMPLE VII

To justify the appropriate wavelength range of excitation and emissionlight, the emission-excitation spectra of aggregate centers in Al₂O₃doped with Mg and C in two different states were obtained (FIG. 6 andFIG. 8). The spectra were obtained using a spectrofluorimeter equippedwith the pulsed EG&G Xe-lamp, two scanning Acton Research spectrographsand a cooled CCD from Princeton Instruments. It was shown that a fresh(or erased) crystal shows an intense green luminescence band in theregion of 520 nm with the excitation band corresponding to the blueabsorption band at 435 nm (FIG. 2). After a writing operation with 440nm pulsed laser, the blue 435 nm absorption band (see FIG. 5) and thegreen emission (see FIG. 6) disappears almost completely and the crystalshows an intensive emission band in the region of 750 nm with excitationbands at 255 nm and 335 nm (FIG. 8) assigned to F₂ ⁺(2Mg)-centers. Bothemission bands: green band at 520 and IR band at 750 nm corresponding toF₂ ²⁺(2Mg)- and F₂ ⁺(2Mg)-centers have a short lifetime of about 9 and83 ns respectively (see FIGS. 4 and 7).

EXAMPLE VIII

Photo-induced transformation of color centers in Al₂O₃:C,Mg utilized forrecording and erasing the data was demonstrated. Exposure of anAl₂O₃:C,Mg crystal having oxygen vacancy defects to high intensity laserlight of appropriate wavelength results in photo-induced conversion ofthe same structural defect from one charged state into another. Forexample, F₂ ²⁺(2Mg)-centers are converted into F₂ ⁺(2Mg)-centers with430 nm illumination (FIG. 5) and may be converted back with 335 nmpulsed laser light. After photochromic transition induced by blue laserlight, Al₂O₃:C,Mg crystals exhibit an absorption/excitation band at 335nm (FIGS. 5 and 6), and a corresponding broad fluorescent emission at750 nm (FIG. 6).

EXAMPLE IX

The evidence of preferred two-photon absorption in aggregate oxygenvacancy defects is provided by quadratic dependence of thephoto-ionization cross-section of these centers versus laser lightintensity (see FIG. 8). The test was performed with 415 nm, 4.5 ns laserpulses of an optical parametric oscillator illuminating a thin, 380 μm,Al₂O₃:C,Mg crystal and recording the decay constant of fluorescence as afunction of laser energy density. The photo-ionization cross-section wasthan calculated as inversely proportional to this decay constant.

Wavelength dependence of a photo-ionization cross-section of F₂ ²⁺(2Mg)centers was measured as a function of wavelength. The peak position of aphoto-ionization cross-section is shifted to a shorter wavelength incomparison with the one-photon absorption/excitation band (FIGS. 3 and9) and is another indication of the 2PA process in Al₂O₃:C,Mg crystals.

EXAMPLE X

A non-destructive readout utilizing one-photon absorption andfluorescent detection scheme was tested. A “write” operation wasperformed on Al₂O₃:C,Mg crystal using Continuum Panther OpticalParametric laser system. The laser system was tuned to generate a signalbeam at 430 nm with the pulse duration of 4.5 ns and 60 μJ/mm² of energydensity per pulse at the sample location. Reading of the written areasusing Type 1 (or so-called “negative” operation) was performed with theblue laser diode from PicoQuant (0.6 mW of average power, 60 ps pulses,and the repetition rate of 20 MHz). Fluorescence at 520 nm was detectedusing a long pass glass filter OG515, high-speed ThorLabs DET210 siliconphotodetector and Tektonix TDS-3054 oscilloscope. A fluorescent signalwith decay time of 9 ns is presented in FIG. 4 and indicates thepossibility to achieve a data transfer rate of up to 100 Mbit/s. Thefluorescent signal of unwritten area of the crystal show intensive 520nm fluorescence. The amplitude of this pulsed signal did not show anydecrease for several hours indicating that there is only one-photonabsorption during “read” operation. For comparison, area of the crystalsubjected to “writing” pulsed 435 nm laser light shows fluorescentsignal equal to only 10% of the signal from unwritten crystal.

EXAMPLE XI

Readout operation utilizing a Type 2 or “positive” fluorescent processwas tested. The “write” operation was performed using the same crystalsample as described in Example II and the same laser system described inExample IX. Reading of the written areas using Type 2 (or “positive”type of operation) was performed with the 335 nm UV beam from the sameOPO laser system (100 nJ/pulse, 4.5 ns pulse duration and 10 Hzrepetition rate). Fluorescence at 750 nm was detected using a long passglass filter RG610 and a silicon photodiode DET-110 from ThorLabs, Inc.and Tektonix TDS-3054 oscilloscope. A fluorescent signal with decay timeof 80±10 ns is presented in FIG. 7. An infrared fluorescence band at 750nm of a bleached (written) crystal (see FIG. 6) has a longer lifetimethan 520 nm green fluorescence of an erased crystal medium but it isstill fast enough for the data transfer rate operation of up to 10 Mb/s.

EXAMPLE XII

FIG. 5 can be used as another illustration of multilevel data storagecapabilities of the utilized optical storage medium based on the inverseproportionality between 435 and 335 nm absorption band intensity as afunction of writing time using 430 nm writing beam. The Al₂O₃:C,Mgcrystal produced according to the Example I was subjected to anincrementing number of 430 nm “writing” laser pulses of the OPO lasersystem described in Example V. Each second of illumination correspondsto 10 laser pulses. Absorption at a 435 nm band associated with the 520nm fluorescent signal reduces as a function of number of writing laserpulses whereas the absorption of a 335 nm band associated with F₂ ⁺(2Mg)and infrared luminescence at 750 nm increases at the same time.

EXAMPLE XIII

A very important feature of photochromic transformations in Al₂O₃:C,Mgcrystals is their high thermal stability. This high thermal and opticalstability of recorded information is attributed to deep traps createdduring crystal growth and is demonstrated by a step-annealing test ofoptical absorption bands in Al₂O₃:C,Mg crystal (FIG. 10). Duringrecording with high intensity blue laser light, 430 nm bands convertinto 335 nm and 630 nm bands [2F₂ ²⁺(2Mg)+2hv→F₂ ⁺(2Mg)+F₂ ³⁺(2Mg)].Reverse transformation of optical absorption bands [F₂ ⁺(2Mg)+F₂³⁺(2Mg)→F₂ ²+(2Mg)] takes place at about 650° C.

The same reverse photochromic transformation and restoration of the 435nm absorption band was achieved by either pulsed 335×20 nm illuminationor more efficient by sequential illumination of the crystal with 215 nmand than with 335 nm laser light corresponding to absorption of F and F₂⁺(2Mg) centers. An original strong absorption at 435 nm and fluorescenceat 520 nm was restored. The inverse photochromic process that convertsF₂ ⁺(Mg)-centers into F₂ ²⁺(Mg)-centers can be performed by two-photonabsorption of laser light within 335 nm absorption band. Completerestoration of original green coloration of Al₂O₃:C,Mg can be achievedby heating the crystal to 650° C.

All documents, patents, journal articles and other materials cited inthe present application are hereby incorporated by reference.

All documents, patents, journal articles and other materials cited inthe present application are hereby incorporated by reference.

It is important to emphasize that the invention is not limited in itsapplication to the detail of the particular material and technologicalsteps illustrated herein. The invention is capable of other embodimentsand of being practiced or carried out in a variety of ways. It is to beunderstood that the phraseology and terminology employed herein is forthe purpose of description and not of limitation.

Although the present invention has been fully described in conjunctionwith the preferred embodiment thereof with reference to the accompanyingdrawings, it is to be understood that various changes and modificationsmay be apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims, unless they departtherefrom.

1. A method of writing information to a data storage medium comprisingthe steps of: providing a luminescent data storage medium comprisingAl₂O₃; and writing said information to said luminescent data storagemedium with an optical source, wherein said information is written tosaid luminescent data storage medium by using a two-photon absorptiontechnique and a photo-ionization technique resulting in removal of anelectron from a color center in said luminescent data storage medium andmoving said electron to a thermally stable trap in said luminescent datastorage medium.
 2. The method of claim 1, wherein said two-photonabsorption technique is a sequential two-step two-photon absorptiontechnique.
 3. The method of claim 1, wherein said two-photon absorptiontechnique is a simultaneous direct two-photon absorption withoutintermediate levels.
 4. The method of claim 1, wherein said luminescentdata storage medium is written to in more than one layer at thedifferent depths inside said data storage medium.
 5. The method of claim4, wherein said optical source emits a laser beam from an opticalread/write head and said optical read/write head incorporating means forspherical aberration compensation allowing for a diffraction limitedspot at a depth of at least 10 microns.
 6. The method of claim 1,wherein said luminescent data storage medium is written to differentmodulation depths to thereby achieve multilevel data storage.
 7. Themethod of claim 1, wherein at least part of the said luminescent datastorage medium is a single crystal Al₂O₃ material.
 8. The method ofclaim 1, further comprising the step of: focusing said optical source toa predetermined depth in said luminescent data storage medium.
 9. Themethod of claim 8, wherein said optical source is focused by moving saidluminescent data storage medium with respect to said optical source. 10.The method of claim 8, wherein said laser beam is focused on saidluminescent data storage medium by adjusting the position of an opticalpick-up head containing said optical source.
 11. The method of claim 1,further comprising the step of: moving said luminescent data storagemedium to a write position prior to said laser beam writing to saidluminescent data storage medium.
 12. The method of claim 1, wherein saidoptical source emits a laser beam having a power density of greater than10³ W/cm².
 13. The method of claim 1, wherein said optical source emitsa laser beam having a power density of at least 10⁵ W/cm².
 14. A methodof writing information to a data storage medium comprising the steps of:providing a luminescent data storage medium comprising Al₂O₃; andwriting said information to said luminescent data storage medium with anoptical source, wherein said luminescent data storage medium comprises:a base material comprising Al₂O₃; a first dopant comprising Mg; and asecond dopant comprising carbon, wherein said luminescent data storagemedium includes a plurality of at least one type of oxygen vacancydefect.
 15. The method of claim 14, wherein said luminescent datastorage medium includes at least one color center having: an absorptionin the region of 435±5 nm, an emission in the region of 520±5 nm and a9±3 ns fluorescence lifetime.
 16. The method of claim 14, wherein saidluminescent data storage medium includes at least one color centerhaving: an absorption in the region of 335±5 nm, an emission in theregion of 750±5 nm and a 80±10 ns fluorescence lifetime.
 17. The methodof claim 14, wherein said luminescent data storage medium includes atleast one color center having: an absorption in the region of 435±5 nm,an emission in the region of 520±5 nm and a 9±3 ns fluorescence lifetimeand at least one color center having: an absorption in the region of335±5 nm, an emission in the region of 750±5 nm and a 80±10 ns lifetime.18. The method of claim 14, wherein said luminescent data storage mediumis written for a write time sufficient to change fluorescence signalamplitude by at least 1%.
 19. The method of claim 14, wherein said laserbeam has a wavelength of 370 to 490 nm, inclusive.
 20. The method ofclaim 14, wherein said optical source emits a laser beam having awavelength of 390 nm.
 21. The method of claim 14, wherein said opticalsource emits a laser beam having a write time in the range of 0.1 ps to1 ms.
 22. The method of claim 14, wherein said optical source emits alaser beam having has a write time of 10 ns.
 23. A method of readinginformation stored on a data storage medium comprising the steps of: (a)exciting a luminescent data storage medium with an optical source tothereby cause said luminescent data storage medium to emit a fluorescentlight signal, wherein said luminescent data storage medium comprisesAl₂O₃ and wherein said optical source emits a read laser beam having awavelength in the range of an absorption band of said luminescent datastorage medium; and (b) measuring said laser induced fluorescence lightsignal from said luminescent data storage medium, to thereby read saidinformation stored on said luminescent data storage medium, wherein step(a) comprises exciting said luminescent data storage medium using aone-photon absorption technique without causing photo-ionization of thestorage centers to thereby cause said luminescent data storage medium toemit a fluorescent light signal and thereby read said luminescent datastorage medium non-destructively.
 24. The method of claim 23 whereininformation from said luminescent data storage medium is read from morethan one layer at the different depths inside said luminescent datastorage medium.
 25. The method of claim 24, wherein step (b) comprisesdetecting said fluorescence signal using a confocal detection technique.26. The method of claim 24, wherein said read laser beam is emitted bysaid optical source disposed in a read/write head and said opticalread/write head incorporates means for spherical aberration compensationallowing for a diffraction limited spot at a depth of at least 10microns.
 27. The method of claim 23, wherein prior to step (a) saidmethod further comprises the step of: writing to said luminescent datastorage medium with a write laser beam.
 28. The method of claim 27,wherein said read and write laser beams have the same wavelength. 29.The method of claim 27, wherein said read and write laser beams havedifferent wavelengths.
 30. The method of claim 27, wherein said read andwrite laser beams are each focused through a lens and said lens is usedfor writing information to and reading information from said luminescentdata storage medium.
 31. The method of claim 23, further comprising thestep of: moving said luminescent data storage medium with respect tosaid optical source and to a read position prior to said read laser beamexciting said luminescent data storage medium.
 32. The method of claim23, further comprising the step of: focusing said read laser beam to apredetermined depth in said luminescent data storage medium.
 33. Themethod of claim 32, wherein said read laser beam is focused by movingsaid luminescent data storage medium with respect to said read laserbeam.
 34. The method of claim 32, wherein said read laser beam isfocused by adjusting the position of an optical pick-up head containingsaid optical source.
 35. The method of claim 23, wherein saidluminescent data storage medium is read for a read time equal to a readlaser beam pulse length and wherein said luminescent data storage mediumis a stationary data storage medium.
 36. The method of claim 23, whereinsaid luminescent data storage medium is read for a read time equal to aratio of a reading spot size with respect to the velocity of saidluminescent data storage medium and wherein said luminescent datastorage medium is a moving data storage medium.
 37. The method of claim23, wherein said read laser beam has a power density that is less thanabout 10 ³ W/cm².
 38. A method of reading information stored on a datastorage medium comprising the steps of: (a) exciting a luminescent datastorage medium with an optical source to thereby cause said luminescentdata storage medium to emit a fluorescent light signal, wherein saidluminescent data storage medium comprises Al₂O₃ and wherein said opticalsource emits a read laser beam having a wavelength in the range of anabsorption band of said luminescent data storage medium; and (b)measuring said laser induced fluorescence light signal from saidluminescent data storage medium, to thereby read said information storedon said luminescent data storage medium, wherein step (a) comprisesexciting said luminescent data storage medium using a simultaneoustwo-photon absorption technique without causing photo-ionization of thestorage centers to thereby cause said luminescent data storage medium toemit a fluorescent light signal and thereby read said luminescent datastorage medium non-destructively.
 39. The method of claim 38, whereinsaid data storage medium is excited by light from said optical sourcehaving a wavelength about two times longer than the wavelength of theabsorption band of the said luminescent data storage medium.
 40. Amethod of reading information stored on a data storage medium comprisingthe steps of: (a) exciting a luminescent data storage medium with anoptical source to thereby cause said luminescent data storage medium toemit a fluorescent light signal, wherein said luminescent data storagemedium comprises Al₂O₃ and wherein said optical source emits a readlaser beam having a wavelength in the range of an absorption band ofsaid luminescent data storage medium; and (b) measuring said laserinduced fluorescence light signal from said luminescent data storagemedium, to thereby read said information stored on said luminescent datastorage medium, wherein said luminescent data storage medium comprises:a base material comprising Al₂O₃; a first dopant comprising magnesium;and a second dopant comprising carbon, wherein said luminescent datastorage medium includes a plurality of at least one type of oxygenvacancy defect.
 41. The method of claim 40, wherein said luminescentdata storage medium includes at least one color center having: anabsorption in the region of 435±5 nm, an emission in the region of 520±5nm and a 9±3 ns fluorescence lifetime.
 42. The method of claim 40,wherein said luminescent data storage medium includes at least one colorcenter having: an absorption in the region of 335±5 nm, an emission inthe region of 750±5 nm and a 80±10 ns lifetime.
 43. The method of claim40, wherein said read laser beam has a wavelength within an absorptionband of Al₂O₃:C,Mg centered at 335±10 nm and wherein said fluorescentlight signal has an emission band having a wavelength range of 620–880nm, inclusive, and being centered at 750±10 nm.
 44. The method of claim40, wherein said fluorescent light signal is excited using light of thewavelength within an absorption band of Al₂O₃:C,Mg and centered at255±10 nm and wherein said fluorescent light signal has an emission bandhaving a wavelength range of 620 nm to 880 nm, inclusive, and beingcentered at 750±10 nm.
 45. The method of claim 40, wherein saidluminescent data storage medium includes at least one color centerhaving: an absorption in the region of 435±5 nm, an emission in theregion of 520±5 nm and a 9±3 ns fluorescence lifetime and at least onecolor center having: an absorption in the region of 335±5 nm, anemission in the region of 750±5 nm and a 80±10 ns lifetime.
 46. Themethod of claim 40, wherein fluorescent light signal has a wavelength of470 and 580 nm, inclusive, and centered at 520±10 nm.
 47. The method ofclaim 40, wherein said read laser beam illuminates said luminescent datastorage medium for the period of time between 1 ns and 10 μs.
 48. Themethod of claim 40, wherein said read laser beam illuminates saidluminescent data storage medium for about 100 ns.
 49. The method ofclaim 40, wherein said laser beam has a read time of 0.1 ps to 1 s,inclusive.
 50. The method of claim 40, wherein said laser beam has aread time of 10 ns
 51. The method of claim 40, wherein prior to step (a)said method further comprises the step of: writing to said luminescentdata storage medium with a write laser beam.
 52. The method of claim 40,wherein said read and write laser beams have a wavelength of 380 to 490nm, inclusive.
 53. The method of claim 52, wherein said read laser beamhas a wavelength longer than said write laser beam and said read laserbeam has a wavelength of about 430 to 490 nm, inclusive.
 54. A method oferasing information stored on a data storage medium comprising the stepsof: (a) providing a luminescent data storage medium comprising Al₂O₃,said luminescent data storage medium having said information storedthereon; and (b) illuminating said luminescent data storage medium withan optical source to thereby erase said information.
 55. The method ofclaim 54, wherein said information is erased from said data storagemedium using a two-photon absorption technique.
 56. The method of claim54, wherein said luminescent data storage medium comprises: a basematerial comprising Al₂O₃; a first dopant comprising magnesium; and asecond dopant comprising carbon, wherein said luminescent data storagemedium includes a plurality of at least one type of oxygen vacancydefect.
 57. The method of claim 56, wherein said luminescent datastorage medium includes at least one color center having: an absorptionin the region of 435±5 nm, an emission in the region of 520±5 nm and a9±3 ns fluorescence lifetime and at least one color center having: anabsorption in the region of 335±5 nm, an emission in the region of 750±5nm and a 80±10 ns lifetime.
 58. The method of claim 57, wherein step (b)comprises illuminating of said luminescent data storage medium with saidoptical source having a wavelength at 335±30 nm and a power densityabove the threshold of two-photon absorption at 10³ W/cm².
 59. Themethod of claim 58, wherein said illumination is accomplished with saidoptical source having a wavelength at 335±30 and is performed afterilluminating said luminescent data storage medium with UV light having awavelength centered at 205±30 nm.
 60. The method of claim 59, whereinsaid UV light is coherent.
 61. The method of claim 59, wherein said UVlight is incoherent.
 62. An apparatus comprising: a luminescent datastorage medium comprising Al₂O₃; and an optical source for writinginformation to said luminescent data storage medium, wherein saidinformation is written to said luminescent data storage medium by usinga two-photon absorption technique and a photo-ionization techniqueresulting in removal of an electron from a color center in saidluminescent data storage medium and moving said electron to a thermallystable trap in said luminescent data storage medium.
 63. An apparatuscomprising: a luminescent data storage medium comprising Al₂O₃; anoptical source for reading information by exciting said luminescent datastorage medium to thereby cause said luminescent data storage medium toemit a fluorescent light signal when information is stored on saidluminescent data storage medium; and measuring means for detecting saidemitted fluorescent light signal, wherein said information is written tosaid luminescent data storage medium by using a two-photon absorptiontechnique and a photo-ionization technique resulting in removal of anelectron from a color center in said luminescent data storage medium andmoving said electron to a thermally stable trap in said luminescent datastorage medium.
 64. An apparatus comprising: a luminescent data storagemedium comprising Al₂O₃; an optical source for reading information byexciting said luminescent data storage medium to thereby cause saidluminescent data storage medium to emit a fluorescent light signal wheninformation is stored on said luminescent data storage medium; measuringmeans for detecting emitted fluorescent light signal; and a secondoptical source for writing information to said luminescent data storagemedium.
 65. The apparatus of claim 64, wherein said first and secondoptical sources are the same.
 66. The apparatus of claim 65, whereinsaid measuring means include a confocal detection means.
 67. Theapparatus of claim 64, further comprising an optical head including saidfirst optical source and said second optical source.
 68. The apparatusof claim 64, wherein said information is written to said luminescentdata storage medium by using a two-photon absorption technique and aphoto-ionization technique resulting in removal of an electron from acolor center in said luminescent data storage medium and moving saidelectron to a thermally stable trap in said luminescent data storagemedium.
 69. The apparatus of claim 64, wherein said luminescent datastorage medium comprises: a base material comprising Al₂O₃; a firstdopant comprising Mg; and a second dopant comprising carbon, whereinsaid luminescent data storage medium includes a plurality of at leastone type of oxygen vacancy defect.
 70. An apparatus comprising: aluminescent data storage medium comprising Al₂O₃; an optical source forwriting information to said luminescent data storage medium; and,compensation means for adaptive spherical aberration compensation ofsaid optical source to allow optical addressing with a diffractionlimited light spot at a depth of at least 10 microns.
 71. An apparatuscomprising: a luminescent data storage medium comprising Al₂O₃; andwriting means for writing information to said luminescent data storagemedium by using a two-photon absorption technique and a photo-ionizationtechnique resulting in removal of an electron from a color center insaid luminescent data storage medium and moving said electron to athermally stable trap in said luminescent data storage medium, saidwriting means comprising a first optical source.
 72. The apparatus ofclaim 71, further comprising: reading means for exciting saidluminescent data storage medium with an optical source having awavelength in the range of an absorption band of said luminescent datastorage medium to thereby cause said luminescent data storage medium toemit a fluorescent light signal via one-photon absorption withoutphoto-ionization of color centers in said luminescent data storagemedium, said reading means including a second optical source; and meansfor measuring said emitted fluorescent light signal.
 73. The apparatusof claim 72, wherein said first and second optical sources are the same.74. An apparatus comprising: a luminescent data storage mediumcomprising Al₂O₃; and an optical source for writing information to saidluminescent data storage medium, wherein said luminescent data storagemedium comprises: a base material comprising Al₂O₃; a first dopantcomprising Mg; and a second dopant comprising carbon, wherein saidluminescent data storage medium includes a plurality of at least onetype of oxygen vacancy defect.
 75. An apparatus comprising: aluminescent data storage medium comprising Al₂O₃; an optical source forreading information by exciting said luminescent data storage medium tothereby cause said luminescent data storage medium to emit a fluorescentlight signal when information is stored on said luminescent data storagemedium; and measuring means for measuring said emitted fluorescent lightsignal, wherein said luminescent data storage medium comprises: a basematerial comprising Al₂O₃; a first dopant comprising Mg; and a seconddopant comprising carbon, wherein said luminescent data storage mediumincludes a plurality of at least one type of oxygen vacancy defect.